rhombus

A quadrilateral in which both pairs
of opposite sides are parallel and all sides are the same length, that is,
an equilateral parallelogram. A rhombus
is also sometimes called a rhomb or a diamond. A rhombus
whose acute angles are 45° is called a lozenge. The diagonals p and q of a rhombus are perpendicular and satisfy the relationship p2 + q2 = 4a2. The
area of a rhombus is given by A = ½ pq.